Olefins are produced by the thermal and/or catalytic dehydrogenation of alkanes. In the thermal cracking processes, alkanes are passed through tubes which are maintained at a temperature which is sufficiently high to cause dehydrogenation of alkanes to alkenes. A by-product of thermal cracking is "coke", which is a material of high carbon to hydrogen atomic ratio. The coke forms a deposit which reduces heat transfer from outside the tubes to the alkane stream passing through the interior of the tubes. A common expedient to reduce coke deposition is to pass steam through the tubes with the alkane stream. The steam reduces the vapour pressure of the alkanes and tends to reduce polymerization side-reactions involved in the formation of coke. Processes wherein alkanes are dehydrogenated to alkenes in the presence of steam are known as steam cracking processes, and are widely used in the chemical industry for the manufacture of ethene and propene, inter alia.